The microphone of a telephone handset responds to both speech signals and to background noise. Such background noise is disturbing not only to the local user but also to a distant party involved in a telephone conversation with the local user. Telephone stations are generally arranged to provide a certain amount of feedback so that conversations sound normal, even though one ear is obviously blocked from receiving the user's voice signals transmitted though the air. Such feedback is known as "sidetone;" and, although sidetone is useful when the user is speaking, it is annoying at a location where background noise is particularly high and the user is only listening. For example, in an effort to hear the conversation, the user may place her hand over one ear to block out room noise - only to have it enter her other ear via the handset microphone and sidetone path. Background noise is particularly offensive during a conference call in which each conferee is exposed to the combined background noise of all conferees.
In some cases, signal-to-noise ratio has been improved through the use of an expander circuit having two gain levels. In the absence of a strong speech signal into the microphone, the gain of the transmitting amplifier is set to its minimum level and thus reduces the noise transmitted to the receiver. When a strong speech signal is detected, the gain of the transmitting amplifier is set to its maximum level so that the speech signal is transmitted to the distant party at the proper level. Such expanders perform reasonably well so long as the user speaks directly into the microphone; however, many users do not, and even make matters worse by positioning the mouthpiece under their chin. In such cases, there may not be sufficient speech energy reaching the microphone to cause the expander to switch properly. Although the switching threshold can be lowered, such lowering decreases the margin against switching due to background noise.
In U.S. Pat. No. 3,889,059 the ambient noise level is used to proportionally decrease the transmitted speech signal level. Although this compensates for the natural tendency of a user to speak more loudly in a noisy environment than in a quiet environment and results in a more constant speech signal level being presented to the transmission facility, it does not address the problem of the transmission of background noise at the location where the user is merely listening.
U.S. Pat. No. 3,963,868 discloses the use of a noise guard circuit whose output is added with the transmit and receive signals so as to prevent ambient noise from influencing the enabled direction of transmission. This particular use of ambient noise measuring equipment is useful in a speakerphone application but, once again, does not address the problem of transmitting ambient noise in a full duplex environment when the local user is listening.
It is therefore an object of the present invention to minimize the transmission of background noise picked up by a telephone handset microphone during the time that the user of the handset is not speaking.
It is another object of the present invention to provide effective expander action when the user is not speaking directly into the microphone.